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Nationally, the number of environmental pollution prosecutions has more than halved over the past five years. There were 555 prosecutions in the 2010/2011 financial year and 234 last financial year. Last year, 38 per cent of cases resulted in convictions compared to 43 per cent in the 2010/2011 financial year.

University of Auckland Associate Professor Kenneth Palmer said he was not surprised the number of prosecutions had dropped.

Dr Palmer's teaching and research interests include environmental law, resource management law, and local government law. He thought people were becoming better educated about environmental responsibilities.

Farmers had proper effluent treatment measures, regional councils were making sure farms were complying with the rules and people weren't polluting the way they used to. Enforcement around the transportation of hazardous substances was also stricter.

"So overall, better standards, better education, safer practices. So that's good news that people are getting the message and are now not trying to get away with things."

Dr Palmer said he did not have concerns over the rate of conviction as identifying offenders could be an issue in pollution cases.

There were also defences if somebody did cause pollution and there was nothing to stop somebody receiving a summons going to council and trying to work out a compromise. Dr Palmer said in many areas where there had been no prosecutions it was likely the level of enforcement was good, breaches were not particularly serious and nothing justified the substantial cost of a prosecution and delay in getting a hearing.